Colorectal Cancer CT Colonography Virtual Colonoscopy

CTC is used for the detection of polyps and colorectal cancer in symptomatic patients, patients with failed colonoscopy (pain, sharp bowel curvatures or bowel stenosis) and in screening. CT of the air- or carbon dioxide-distended colon is performed in supine and prone positions, as some bowel segments may be collapsed in one position. This dual positioning also assists movement of residual fluid and stool that may obscure the bowel surface. The use of oral iodine contrast media (e.g. Gastrografin) has facilitated identification of submerged lesions as well as tagging of residual stool. The use of intravenous contrast medium has no clear advantage for polyp detection and may even be counterproductive with the use of oral fluid tagging.

The main purpose of CTC is to detect clinically significant lesions such as colorectal carcinomas and large adenomatous polyps, which are its precursors. As adeno-matous and nonadenomatous lesions cannot be discriminated with CTC, the primary focus of most studies is polyps > 10 mm, as the likelihood of adenomas and malignancy is size-related. Although the primary interest is in polyps > 10 mm, polyps ranging in size from 5-9 mm are also considered relevant.

In symptomatic populations CTC is an accurate technique with a detection sensitivity for colorectal cancer of 95.9% and a specificity of > 99% [17]. For patients with polyps > 10 mm, sensitivity is 85-92.5% and specificity 95-97.4%, while for patients with polyps > 6 mm, sensitivity is 70-86.4% and specificity 86.1-93% [17, 18]. Intravenous contrast medium can be used for the detection of lymphadenopathy and liver metastases in symptomatic patients in one position.

In colorectal cancer, extension outside the colon, such as infiltration of pericolic fat and loss of fat planes between the colon and adjacent organs, can be readily evaluated. Pathways of lymph node metastases can be predicted based on the site of the primary tumor. Complications of primary colonic malignancies, such as obstruction, perforation, and fistula can be visualized with CT.

The liver, the primary site for distant metastases, needs to be examined for the presence of colorectal liver metastases. CT and MRI are the principal diagnostic techniques for detection of liver metastases, with an increasing role of fluorine-18-fluorodeoxyglucose (FDG) positron emission tomography (PET). FDG PET has significantly higher sensitivity on a per-patient basis, compared with that of CT and MRI, but not on a per-lesion basis [19]. Sensitivity estimates for enhanced MR imaging (super-paramagnetic iron oxide [SPIO] or gadolinium) are significantly superior to those for helical CT with 45 g of iodine or less.

Data on CTC in surveillance/screening of populations are still limited. The sensitivity in patients with polyps > 10 mm varies considerably (55-94%), while specificity is high (92-96%) [20]. The studies differ considerably in methodology, including prior reading experience, review methods (two-dimensional [2D] or three-dimensional [3D]), bowel preparation and reference standards. All these factors may have contributed to the differences seen.

CTC examinations have traditionally been performed after extensive bowel cleansing, a major deterrent for full structural colon examinations. Several studies have demonstrated the feasibility of CTC after limited bowel preparation. A low-fiber diet and the use of oral contrast medium has been shown to be sufficient for good CTC results for the detection of colorectal cancer and polyps.

Pitfalls include untagged stool (inhomogeneous, moves in position), complex folds (3D is helpful), extrinsic compression (2D is helpful), ileocecal valve (combined 2D and 3D evaluation), while flat lesions can be difficult to detect. Evaluation of the sigmoid can be difficult in patients with extensive diverticulosis and muscular hypertrophy. Collapsed bowel segments should be scrutinized in the other position. It is important to report the quality of the examination (distension, tagging) and whether a colonoscopy is indicated.

For screening purposes, radiation exposure is a drawback of CT. MRI can be used as an alternative to CT, although this is less common. MR-colonography can be performed after limited bowel preparation, however the optimal MR protocol has not been determined.

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